Metal guardrails are known which are placed along roads and which constitute safety barriers. Safety guardrails made of wood are also known.
U.S. Pat. No. 2 085 058 (Wood), U.S. Pat. No. 1 493 088 (Van Epps), and U.S. Pat. No. 3 989 226 (Burgess) describes safety barriers including rails made of rectangular section wood.
European patent application No. 0 184 525 (85.420202.5) (Eynard) describes road safety guardrails comprising a rail made of wooden logs assembled end-to-end by T-shaped metal reinforcements and by wooden posts.
European patent application No. 0 228 334 (86.430049.6) (Compagnie Francaise des Etablissements Gaillard) also describes road safety guardrails comprising roundwood posts and roundwood rails assembled end-to-end by pairs of metal straps which advantageously include spikes on their inside faces for penetrating into the rails.
The present invention relates to improvements to the safety guardrails described in said published European patent application No. 0 228 334.
The problem to be solved is the following:
Wooden guardrails must satisfy specifications which require assembled rails to have a given traction strength, for example a strength of 200 kilonewtons (i.e. about 20 (long) tons), so that in the event of an impact from a vehicle, the rails do not run the risk of coming apart, thereby allowing the vehicle to pass through them.
Statistical testing performed on samples of logs held together by pairs of straps which are interconnected by bolts have shown that the horizontal diametrical plane in which the bolts are located constitutes a plane of weakness and also that the measured traction strengths have a wide degree of dispersion about the mean value, namely about 10%. As a result it is difficult to meet traction strength specifications without excessively increasing the diameter of the logs from which the rails are made up.
The problem is thus to improve safety barriers including at least one rail made up of roundwood logs assembled end-to-end in such a manner as to increase the static traction strength of the connections between logs by using reinforcing means which are simple and cheap, so as to enable wooden guardrails to remain competitive in price.
In addition to their static traction strength, road safety barriers must also meet specified dynamic behavior requirements. In the event of impact from a vehicle, the connections between the component parts must enable a large amount of overall lengthening to take place without breaking so as to absorb the kinetic energy of the vehicle.
An object of the present invention is therefore to improve the end-to-end connection between logs in order to confer high static traction strength thereto while still enabling considerable elongation to occur at the connections situated on either side of an impact to occur in the event of impact from a vehicle.
Belgium patent No. 883 394 (Yves Durand) describes end-to-end connections between rectangular section beams comprising both assembly bolts passing through a plurality of juxtaposed beams or passing through a single beam and two connection straps, together with reinforcing members constituted by two plates which are clamped against the sides of each beam at the ends thereof by threaded rods passing through the beams and running perpendicular to the assembly bolts. Such reinforcing members compress the wood situated in the zones through which the assembly bolts pass.
Reinforcement by such plates is not suitable for interconnecting logs in a safety guardrail since such reinforcement necessarily requires plates and rod ends to project proud above and below the logs and this would be highly dangerous for the safety of passengers in the vehicles. In addition, the plates which compress the wood cannot be used out-of-doors because the wood will swell due to variations in humidity and this will loosen the clamping plates and make them ineffective.
Finally, when the plates are clamped together to compress the wood, the assembly becomes rigid and in the event of impact from a vehicle, it is capable of elongating by a small amount only.